There is a desire on the part of vehicle manufacturers to increase the electrification of vehicle auxiliary loads by reducing the number of engine accessories that depend directly on the fueled-engine as a prime mover. Examples include the vehicle's power steering pump, hydraulic drives, engine cooling fan, air conditioning compressor, oil and coolant pumps, and air compressors. An advantage of accessory electrification is reduced engine loading, which facilitates greater engine performance, increased flexibility in locating the accessories, reduced fuel consumption, more efficient accessory operation, and reduced vehicle emissions.
In addition to engine accessories, many vehicles include ancillary electrical accessories directed to the mission of the vehicle and/or comfort of the vehicle's occupants. For example, an emergency vehicle includes exterior emergency lighting, heating, ventilation and air conditioning (“HVAC”), interior lights, radios and medical equipment. Many trucks, such as long-haul transport tractor-trailer semis, include radios, exterior and interior lights, and a sleeper compartment having a variety of household appliances such as televisions and microwaves that are operable from AC power supplied by a DC-to-AC inverter.
Vehicle electrical systems typically include one or more batteries comprising an electrical power supply, with an engine-driven alternator being employed to augment and charge the battery. A particular problem of such systems is that the loading of the system and, consequently, the alternator output constantly changes as various engine and ancillary accessories are switched on and off. The output of the alternator, which varies in proportion to the load on the electrical system and the discharge of the battery, responds relatively slowly to the changes in loading due to the inductive time constant of the field and stator coils of the alternator. This can adversely affect the voltage regulation of the vehicle's electrical system. For example, if a high-power exterior light is switched on, the increased loading of the electrical system causes the electrical system voltage to decrease until the alternator eventually responds by increasing its output voltage. Conversely, when a load is switched off, the electrical system voltage may surge to a higher level than is desirable until the alternator output is reduced. The slow response of the alternator to changing load conditions thus results in relatively poor electrical system voltage regulation. This problem is exacerbated when a number of accessories are randomly switched on and off at any given time. There is a need to improve an alternator's response to varying load conditions within a vehicle's electrical system.